Photomechanical printing



Patented Dec. 5, 1933 UNITED STATES PHOTOMECHANICAL PRINTING (POSITIVE COLLOID RELIEF) Ralph Emerson Hurst and Charles Edmund Meulendyke, Rochester, N. Y.

No Drawing. Application May 7, 1932, Serial 11 Claims.

This invention relates especially to the preparation of metallic sheets supporting a colloid layer containing a sensitive silver salt with or without an actinic-light restraining agent and to the subsequent processing of such sheets into relief, planographic and intaglio plates for monochrome and polychrome printing.

Within the scope of these improvements and within the art of photo-mechanical printing we intend to include new and useful materials and the processing of the same into relief printing plates as for line and half-tone engraving, intaglio printing plates as for photogravure and rotogravure, and planographic printing plates as for photo-metallography, and offset printing.

Generally speaking in the preparation of a photo-mechanical printing plate it is necessary to coat a metallic sheet with a colloid layer containing a light-sensitive salt. In the present state of the art this layer generaly consists of a bichromated glue or other colloid. Upon exposure under a photographic negative the glue becomes insoluble where the light penetrates. Upon washing in water the unafiected glue dissolves out leaving a positive colloid relief.- This colloid relief can then serve as a resist so that the metal can be etched where unprotected by the resist. The metallic plate so etched when properly inked can serve as a printing plate.

There are objections to a'bichromated colloid appreciated by those skilled in the art. A blchromated colloid layer rapidly deteriorates, requiring preparation shortly before using. It has too little sensitivity for direct exposure in the camera and even requires several minutes exposure to daylight in contact printing. The essentially hand-preparation of metallic plates so sensitized is labor-consuming and time-consuming. A ready-prepared plate of good keeping quality would tend to be cheaper, of better and more uniform printing quality, and of greater freedom from defects, derived from the ordinary advantages of quantity-production. Also plates capable of direct exposure in the camera would tend to simplify existing processes. In its favor it may be said that a bichromated colloid can yield an image of high quality and high resolution as evidenced by clean separation of half tone dots.

It is the purpose of this invention to prepare a metallic sheet supporting a light-sensitive colloid layer of good keeping quality, of sufiicient sensitivity for direct exposure in the camera and capable of being directly transformed into a printing plate which in quality can compare most favorably with that afforded by a wet collodion negative printed onto a sheet sensitized with bichromated glue. In referring to a metallic sheet we intend to include metal in the form of a sheet, plate, cylinder, foil, deposition or other form. We will now describe in a general way the steps by which such improvements are accomplished. Later We will show the practicality of such improvements as carried out in a typical branch of the photo-mechanical printing industry.

It is a fact that what is known as a photographic emulsion can be coated on a glass plate and that such coated plate can be made with good keeping quality and with sufficient sensitivity for direct exposure in the camera. A process gelatin dry plate is an example. A photographic emulsion consists of a sensitive silver salt incorporated in a colloid medium which is generaly gelatin. In referring to a sensitive salt we intend to include those silver salts capable of exposure and development which are ordinarily coated on photographic plates, films and papers, and in referring to colloid we intend to include not. only gelatin but other equivalent colloids.

It is also a fact that a developed silver image in a gelatin layer can be treated with an oxidizing agent such as hydrogen peroxide or persulfate to soften or to dissolve out the gelatin under the se lective control of the silver image, that is to say, the' gelatin immediately adjacent to the silver particles becomes soft or under strong conditions of oxidation completely dissolved out. Thus a photographic image can be converted into a colloid print of differentially softened gelatin or preferablyinto a colloid relief. It is apparent that by this process in which the developed silver and contiguous gelatin are washed away, direct exposure in the camera will produce a positive colloid relief. In our companion application filed May 16, 1932, Serial No. 611,646 we propose to treat the developed silver image with a bichro-' mate in solution in which process the gelatin surrounding the silver particles becomes insoluble producing a diiferentially hardened colloid print or, in case the soluble gelatin is washed loo away, a colloid relief. In this case direct exposure in the camera will produce a negative colloid image. In both applications the same emulsion-coated metallic sheets are employed. They are fully claimed in the companion application 105 but are described in this specification for the proper understanding of this invention.

The general method which we propose is to coat a metallic sheet with a photographic emulsion of sufficient sensitivity for contact printing or for up direct exposure in the camera, to expose and develop the silver image, to dissolve out the gelatin by -a suitable agent under the selective control of the silver image thus forming a colloid relief which can serve as a resist directly supported by the metal, and to subsequently convert to a printing plate by appropriate means. The accomplishment of making and processing metallic sheets directly coated with emulsion presented diiiiculties to be overcome. A photographic emulsion coated directly upon the ordinary materials of printing plates such as zinc and copper gave rapid deterioration of the emulsion. The well-known types of sensitizing generally employed in photo-mechanical work such as bichromated glue and sensitive bitumen can be coated directly on metallic surfaces without dan ger of metallic contamination. The particular type of sensitizing which we propose has decided advantages in speed and color sensitivity but its direct coating upon metal has hitherto been associated with metallic contamination. To avoid this it has been proposed to protect such sensitizing by interposing a layer of lacquer or varnish between the sensitizing and the metallic surface and also to treat the metallic surface so as to form a protecting layer of metallic oxides or salts. The first method greatly increases the number of steps and the difficulties of processing. The second method furnishes a very doubtful means of protection and a troublesome layer in processing. We have been succesful in plating a basic metallic sheet with a metallic layer which is truly inert to a directlysupported emulsion layer. An example is a copper base sheet which is plated with silver, nickel, or chromium, and subsequently coated with emulsion. We have thus produced a metallic sheet which can be directly coated with emulsion without the contamination hitherto associated with directly coated metal. This metallic protecting layer is integral with the metallic base sheet and does not interfere with any subsequent etching of the metal. It has all the advantages of a protecting layer without the disadvantages of the above-mentioned layer of lacquer or varnish. The surface of such protecting metallic layer is preferably slightly grained to promote the adhesion of the sensitizing layer. A well-grained surface is preferably in preparing planographic printing plates by this method. The use of a subbing to promote the adhesion of the sensitizing layer to the metallic surface is preferably to be avoided, but we intendany use of such subbing to be included within the meaning of a metallic surface directly supporting a sensitizing layer. The method of electroplating is preferably used in the formation of the protecting metallic layer.

At this point we will describe a manner in which a suitable source of emulsion can be obtained. An emulsion such as is suitable for a process dry plate is suitable for performing the operations herein described. There are various the emulsion absorbed about ten times its weight in water, then scraping oh the emulsion with a knife or straight edge and dissolving at a temperature around 100 Fahrenheit, a convenient source of emulsion was available.

Up to this point we have described the preparation of metallic sheets which have been plated with metal as a means of protecting the emulsion from deterioration. Also we have described a simple way of securing a source of prepared emulsion.

A convenient method of coating a metallic plate is to take a definite amount of the melted emulsion and pour upon the slightly warmed plate, tilting the plate in different directions until the emulsion has evened out, then placing upon a chilled marble slab to set the emulsion, after which the plate can be dried in a gentle current of air. Another way is to pour on an excess of emulsion and to drain 01f in the fashion of preparing a wet collodion plate. It is obvious that these operations must be carried on in a photographic dark room.

Assuming that we have coated a metallic sheet with an emulsion as described above an exacting and typical method of employing the improvements embodied in this invention is to expose the sheet to suitable copy in a camera behind a half-tone screen. A 60 line screen and a 133 screen afford a practical range. We found that thinner layers of emulsion than are ordinarily coated on normal dry plates were conducive to best results. On the other hand on coating the emulsion too thinly, the high-lights tended to be over exposed before the shadows were properly rendered, giving halation and poor resolution of dots. We found that excellent results could be obtained when the thickness of the emulsion layer and the exposure were properly correlated and when the physical hardness of the gelatin and the oxidizing potency of the gelatin-dissolving solution were properly correlated. Further we found that the expedient of adding a light-restraining agent to the emulsion aided the control of the exposing and processing operations. An example of such agent is the yellow dye tartrazin which can be added in the proportion of a trace to .2 gram of dye to 10 grams of the melted emulsion described above. However either with or without addition of the dye we obtained the conditions whereby upon exposure in the camera and processing, the residual gelatin dots in both high-lights and shadows were well defined and remained attached to the support, and whereby the spaces between such dots corresponding to the dissolved out gelatin were clean bare metal. In the case of contact printing such as exposing under a half-tone negative somewhat different conditions obtain than for direct exposure through a half-tone screen in the camera. Due to excessive spreading of the image upon exposure in contact printing we found the use of a light-restraining agent necessary for satisfactoryresults. We believe that we are the first to propose coating a 001- loid layer containing a sensitive silver salt with or Without an actinic-light restraining agent directly upon a metallic sheet consisting of a base sheet plated with a metallic layer truly inert to the emulsion. We believe that we are first to prepare and process such a material into a metallic vehicle supporting a colloid image or relief, and to transform such material into a photo-mechanical printing plate.

An emulsion such as used above which has not 15 p 3% solution of hydrogen peroxide.

been made color-sensitive is largely sensitive to blue and violet light and a dye such as tartrazin which does not harm the emulsion can be added to it. The yellow color acts as a filter to hold back the penetration of the blue and violet light to which the emulsion is sensitive. The presence of a light-restraining agent prevents halation, holds back the high-light exposure to the benefit of the shadows, localizes the developed silver for easier softening or solution of the gelatin, and gives a print and colloid relief of sharp definition. In the case of an emulsion which has been made sensitive to some other color such as green or red it would of course be necessary to employ dyes which would absorb the green or the red to get an equivalent light-restraining action. For example with red sensitive emulsion, napthol green would tend to absorb red radiation.

In the processing of the exposed sheet any suitable developer can be used. A metol-hydroquinone developer such as is recommended for a process dry plate is suitable. Development should be followed by a thorough rinsing. It is preferable that the gelatin should be physically hardened. This can be taken care of by adding a suitable hardener such as chrome alum to the emulsion at the time of coating, or after development the gelatin can be hardened in any suitable bath. The washed sheet, preferably after dry ing, is then placed in the oxidizing bath. A convenient bath consists of a commercially prepared The oxidizing potency of this bath can be increased by adding a trace of copper chloride. In this bath the gelatin corresponding to the developed silver image is dissolved out while the remaining gelatin retains its original insoluble condition. The reaction is preferably completed in from two to five minutes. It is unnecessary to fix out the residual silver salts. When properly carried out in the half-tone process the original developed silver dots will be destroyed. Corresponding to these dots there will be clean clear spaces of netal between the unaffected gelatin dots. Where bare metal is exposed between the gelatin dots the metal can be etched by some suitable agent. Ferric chloride can be used to etch the protecting layer of nickel dilute hydrochloric acid for etching chromium, and dilute nitric acid for etching silver.

As a specific example of the application of the new and useful improvements embodied in this invention we may take the case of preparing a half-tone relief plate by photo-engraving. Normally, in the present state of the art, the follow- T ll ing steps are required before handing the metal sheet to the etcher:

l. Coating of a wet-collodion plate.

2. Exposing in camera behind a half-tone screen.

3. Developing the negative.

4. Stripping and laterally reversing the negative.

5. Preparation of metal ehroznated glue.

e. Exposing the metal plate.

'l. Developing the colloid relief in water.

There is involved along series of steps. There coated with biare the difficult and time-consuming steps oi pine llrere prepared metallic plate supporting a colloid layer containing a sensitive silver salt, as described in this specification, we can expose the same directly in the camera behind a half-tone screen and obtain a photographic negative which can be processed directly into a positive colloid relief in correct lateral position ready for the etcher. There is one photographic image instead of two, no hand-preparation ofmaterials, no long exposure and no stripping-reversing operations.

It will thus be seen that we have described materials and processes with all necessary details, embodying the principles and attaining the objects and advantages of the invention. Since many matters of treatment, manipulation, selection and proportion of ingredients, succession of steps and other details may be variously modified without departing from the principles involved, we do not intend any limitation to such details excepting so far as set forth in the appended claims.

' What is claimed is:

1. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid relief including the steps of exposing a basic metallic sheet whose surface is plated with a. metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, and dissolving out the colloid by a suitable agent under the selective control of the silver image.

2. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid relief including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, and dissolving out the colloid by a suitable oxidizing agent under the selective control of the silver image.

3. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid relief including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, and dissolving out the colloid by a suitable oxidizing agent under the selective control of the silver image, such agent containing hydrogen peroxide.

4. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid relief including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, and dissolving out the colloid by a suitable oxidizing agent under the selective control of the silver image, such agent containing a persulfate.

5. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid print including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, and softening the colloid by a suitable agent under the selective control of the silver image.

6. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a photographic colloid relief including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer inert to and directly supporting a colloid layer containing a sensitive silver salt and an actinic-light restraining agent, developing the silver image, and dissolving out the colloid by a suitable agent under the selective control of the silver image.

7. In the art of photo-mechanical printing, the method of producing a metallic sheet supporting a, photographic colloid print including the steps of exposing abasic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing ,a sensitive silver salt and an actinlc-light restraining agent, developing the silver image, and softening the colloid by a suitable agent under the selective control of the silver image.

8. In the art of photo-mechanical printing, the method of producing a printing plate including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, dissolving out the colloid by a suitable agent under the selective control of the silver image, and etching the metallic surface by a suitable agent under the selective control of the colloid image.

9. In the art of photo-mechanical printing, the method of producing a printing plate in cluding the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt, developing the silver image, softening the colloid by a suitable agent under the selective control of the silver image, and etching the metallic surface by a suitable agent under the selective control of the colloid image.

10. In the art of photo-mechanical printing, the method of producing a printing plate including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt and an actinic-light restraining agent, developing the silver image, dissolving out the colloid by a suitable agent under the selective control of the silver image, and etching the metallic surface by a suitable agent under the selective control of the colloid image.

ll. In the art of photo-mechanical printing, the method of producing a printing plate including the steps of exposing a basic metallic sheet whose surface is plated with a metallic layer truly inert to and directly supporting a colloid layer containing a sensitive silver salt and an actiniclight restraining agent, developing the silver image, softening the colloid by a suitable agent under the selective control of the silver image. and etching the,metallic surface by a suitable agent under the selective control of the colloid image.

RALPH EMERSON HURST. CHARLES EDMUND MEULENDYKE. 

